Commercial screening machines generally have a removable top cover which extends over a frame in which the screen is mounted. The cover encloses the material being screened, preventing it from being shaken off the screen, and minimizes dust. From time to time it is necessary to remove the cover, for instance to change or replace the screen. Because the frame and screen are shaken with substantial force in operation, the cover is typically clamped to the frame for movement with it. The particulate material to be screened is fed through the cover by an inlet chute which discharges it onto the screen.
Various forms of cover hold-down clamps have been proposed specifically for use on screening machines, including manually operated over-center hold-down clamps, for example of the type shown in Nolte U.S Pat. No. 3,433,357. In the use of such clamps, a clamp arm is manually engaged with the cover or frame or other member to be clamped and an arm is pulled from one side of a center position to the other side, so as to draw together the two members to be clamped. Such manual clamps provide a strong, inflexible clamping force. In order to provide an approximately uniform clamping force at the several clamps around the periphery of the frame (so that the cover is not held too tightly at one area and too loosely at another), each clamp must be manually turned or adjusted to provide roughly the same mechanical clamping force. In a large screening machine there may be as many as 16 clamps around the frame. When several such clamps must all be set and adjusted for uniform force, it often happens that the force of the first-set clamps is changed by the later-set clamps so that it is necessary to go back and readjust the first clamps. Such individual and repetitive adjustment requires substantial time, being done largely by trial and error. Moreover, the screws of such clamps over time become clogged with dust from the material being screened and tend to "gall" or seize so that they cannot be easily turned and adjusted.
In some applications screening machines are subjected to substantial heat in use, as for example when a hot material is being screened. Thermal expansion of the frame and/or cover elements caused by such heat puts a substantial load on a rigid mechanical clamps, making them difficult to open. In some circumstances the force of thermal expansion on a tightly set up clamp can even warp or deform the clamp.
Billstrom Pat. No. 2,776,854 teaches a hydraulic cylinder which when pressurized applies a clamping force to hold a flange against an adjacent surface. The application of pressure to the cylinder also swings a latch into clamping position. This is convenient because the latch automatically engages and disengages the clamping surface upon actuation and release of pressure.
Contastin Pat. No. 4,093,176 shows an air pressure operated clamp in which application of pressure irto a bellows moves a swingable latch member into position to engage beneath a member to be clamped. Upon release of air pressure the clamp is swung to open position by a compression spring. The position at which this occurs is not easily changed, being determined by the fixed geometry and the spring strength.